Chapter 15

Assume that genes A and B are linked
and are 50 map units apart. An animal heterozygous at both loci is crossed with
one that is homozygous recessive at both loci. What percentage of the offspring
will show phenotypes resulting from crossovers? If you did not know that genes A
and B were linked, how would you interpret the results of this cross?

Answer:

: Fifty percent of the offspring would show
phenotypes that resulted from crossovers. These results would be the same as
those from a cross where A and B were not linked. Further crosses
involving other genes on the same chromosome would reveal the linkage and map
distances.

A space probe discovers a planet inhabited by
creatures who reproduce with the same hereditary patterns seen in humans. Three phenotypic characters are
height (T = tall, t = dwarf), head appendages (A =
antennae, a = no antennae), and nose morphology (S = upturned
snout, s = downturned snout). Since the creatures are not
"intelligent," Earth scientists are able to do some controlled
breeding experiments, using various heterozygotes in testcrosses. For tall
heterozygotes with antennae, the offspring are: tall-antennae, 46;
dwarf-antennae, 7; dwarf-no antennae, 42; tall-no antennae, 5. For
heterozygotes with antennae and an upturned snout, the offspring are: antennae-upturned
snout, 47; antennae-downturned snout, 2; no antennae-downturned snout, 48; no
antennae-upturned snout, 3. Calculate the recombination frequencies for both
experiments.